In wireless communications, multiple-input and multiple-output (MIMO) technology involves the use of multiple antennas at both the transmitter and receiver to improve communication performance. MIMO technology offers significant increases in data throughput and link range without additional bandwidth or transmit power via higher spectral efficiency and link reliability or diversity. One of the common functions in MIMO technology is precoding, which is a type of beamforming that leverages spatial diversity. In single-layer beamforming, the same signal is emitted from each of the transmit antennas with appropriate phase and/or gain weighting such that the signal power is maximized at the receiver input. The benefits of beamforming are to increase the received signal gain, by making signals emitted from different antennas add up constructively, and to reduce the multipath fading effect. When the receiver has multiple antennas, the transmit beamforming cannot simultaneously maximize the signal level at all of the receive antennas, and multi-layer beamforming is used. In general, precoding requires knowledge of channel state information (CSI) at the transmitter. MIMO technology is typically combined with orthogonal frequency division multiplexing (OFDM) or orthogonal frequency division multiple access (OFDMA) modulation in wireless communication systems.
In addition to spatial diversity, macro-diversity is another kind of diversity scheme where several receiver antennas and/or transmitter antennas are used for transferring the same signal. With macro-diversity, the distance between the transmitters is much longer than the wavelength, as opposed to micro-diversity where the distance is in the order of or shorter than the wavelength. Therefore, in a cellular network, macro-diversity implies that the antennas are typically situated in different base stations. The aim of macro-diversity is to combat fading and to increase the received signal strength and signal quality in exposed positions (i.e., cell-edge) in between the base stations.
To improve network throughput, especially cell-edge throughput, multi-BS MIMO techniques are supported in cellular OFDM/OFDMA networks. With multi-BS MIMO, all neighboring base stations cooperate with each other and use the same configuration such as modulation and coding scheme (MCS) level, MIMO configuration, and other system parameters. Antennas from all neighboring base stations thus act like a “big” MIMO. While each base station uses precoding to leverage spatial diversity, mobile stations located at cell-edge enjoy macro-diversity by multi-BS collaborative precoding. Precoder selection, however, remains as a challenge in multi-BS MIMO because signals from different base stations are not always coherently combined. Solutions are sought to compensate phase and amplitude differences among signals from cooperative base stations such that the received signal strength and quality can be maximized.